Pool Heating System

ABSTRACT

The pool heating system includes a pool containing a large volume of pool water and an associated building. The building having an attic space over a living space. A heat-exchanger is positioned outside of the living space of the building. The heat exchanger exchanges heat from the attic into pool water that is circulated through the heat exchanger. Any pool water that leaks from the heat-exchanger does not leak into or onto the living space.

This application claims priority and the benefit under 35 U.S.C. §119(e) from U.S. provisional patent application 61/138,764 for “Devices to Reduce the Risk of Water Damage from Air-To-Water Heat Exchangers Installed Within Attics” filed Dec. 18, 2009, which is hereby incorporated by reference.

The disclosure relates generally to heat recovery systems and pool heaters, and in particular to a heat recovery system used to recover heat from an attic space to heat a swimming pool.

BACKGROUND

One of the primary expenses involved with owning a swimming pool 118 is the fuel or electricity required to keep it warm in order to have a significantly long swimming season. Solar heating is an effective alternative that provides heat at a relatively modest cost. The warming of swimming pools 118 is one of the most practical and successful applications of solar energy. For this use of solar energy, the often intermittent presence of direct sunlight does not create difficulties or inconvenience because swimming pools 118 are sizable thermal reservoirs. One or two days of cloudy weather does not usually cause the pool temperature to become uncomfortable. The main obstacle to a more widespread adoption of solar pool heating is the initial cost of the equipment. The primary factor determining the expense involved with most solar heating and power systems is that they require a large amount of material in order to present a large surface area for insolation (exposure to sunlight).

A typical solar panel consists of water passages bonded to a sunlight absorption surface that is covered by transparent glazing. The technology is not complicated but it usually requires a lot of copper and glass. A disadvantage of glazed solar collectors is that they may be heavily damaged by hail storms. More rugged, hail resistant, solar panels can be made, but at greater expensive.

For swimming pool applications, solar panels without glazing are very common. These collectors consist only of flexible black plastic or rubber panels with internal water passages. This less expensive kind of solar collector will have somewhat lower performance than a glazed collector particularly during windy weather or when the ambient air is cold. Plastic and rubber are generally not durable when exposed to sunlight especially when compared to a metal surface. Such plastic panels very often require a significant repair after five years and replacement after ten years of use. The sizing rule of thumb commonly used by solar panel vendors recommends a collector panel area of to 120% of the swimming pool free surface area. Purchasing enough plastic panels to meet this criterion for a moderately sized pool 118 costs at least several hundred to a thousand dollars. For a 600 ft2 pool surface, buying 50% of this area in solar panels costs $1200, and purchasing 120% of the pool area would cost $2830 according to a current swimming pool supply catalog. A fixed solar panel (without a sun tracking mechanism) has to be oriented properly to maximize the amount of insolation it receives. Accordingly, such a panel may be at peak effectiveness during only a few hours of a day. Moreover, in some cases there may not be a suitable expanse of roof surface 24 in the proper orientation for which to mount a solar collector. Another disadvantage of roof-mounted solar panels is the necessity of taking them down and then reinstalling them whenever the roof shingles are replaced. Another issue is that some homeowners may feel that solar panels excessively detract from the aesthetics of a homestead 56. Furthermore, a homeowner may be far more concerned with how solar panels will be accepted by neighbors. Roof-mounted panels will remove some of the load on a house's cooling system by keeping portions of the roof surface 24 from getting hot. However, in most cases solar panels will only cover a small fraction of a roof surface 24

Another technique for warming a swimming pool is to use a translucent plastic pool surface cover with encapsulated air pockets. Such solar blankets or “bubble wrap” covers help reduce evaporation and increase solar gain, but they are inconvenient to stow when the pool is to be used. Pool cover reels can make stowage much easier, but these reels typically cost hundreds of dollars. Some swimming pools 118 have irregular shapes which make a pool cover and reel particularly impractical. Furthermore, the pool blanket covers tend to degrade from sunlight exposure. In addition, a pool cover cannot contribute heat while people are swimming in the pool 118. Accordingly, frequent swimming during the afternoon hours could significantly detract from the benefits of a pool cover.

Fuel-burning pool heaters are still the most common type of pool heater in use. Fuel-burning heaters tend to have a much greater heat output rate than solar systems. For example, natural gas pool heaters typically have heating outputs in the range of 100,000 to 400,000 BTU/hr. To put this quantity in better perspective, a 50-gallon domestic water heater produces about 40,000 BTU/hr and the typical output of a residential furnace is about 50,000 BTU/hr. Accordingly, such combustion pool heaters are often capable of raising the temperature of the pool water 5.6 to 11.1° C. (10 to 20° F.) within the course of a day. However, the cost associated with such a rapid warm-up makes this practice seem extravagant. Moreover, due to the finite supply, fossil fuels will become increasingly scarce and expensive. Hydrocarbon-burning heaters release carbon dioxide into the atmosphere contributing to harmful global climate change. Unlike roof-mounted solar collectors, fuel-burning heaters do not remove any of the load on a house's cooling system. Another shortcoming of natural gas swimming pool heaters is how quickly they rust and corrode. This can be attributed to fact that the chemical processes of corrosion and rusting will be accelerated with an increase in temperature. The metal parts within a combustion pool heater are exposed to a much higher level of temperatures than what is encountered by an ordinary flat plate solar collector. Combustion heaters must have safeguard devices such as flame detectors and fusible links in order to prevent the hazards of fire, explosion or incomplete combustion. Such failsafe devices are commonplace for fuel-burning appliances but the necessary additional parts add to the expense and service requirements of a heater.

As an alternative to solar panels, some swimming pool heaters on the market, such as the SolarAttic PCS2, feature an air-to-water heat exchanger 20 coupled with a fan 22 that is hidden within the attic 18. These systems exploit the heat present within an attic space 18 during the warmer parts of the year. Like solar panels, attic-mounted fan-coil units 88 have been demonstrated to be an effective means to warm a swimming pool 118 at a very low operating cost. Attic heat recovery devices are effective in removing a sizable portion of the overall cooling load on the house by substantially reducing the temperature difference that drives the heat transfer between the attic and the living space 16. Heat is not only conducted from the attic 18 to the living space 16 below through the ceiling partition, but also through the walls of the air conditioning ducts within the attic 18. Air conditioning ducts generally have a much thinner layer of insulation (R value range of 2 to 4) than the layer over the ceiling (R value range of 11 to 38). For the cool air supply ducts, the temperature difference between the attic air and the air within the duct is 8.3 to 11.1° C. (15-20° F.) larger than the temperature difference between the attic 18 and living space 16. Even a well-insulated attic 18 often has gaps or breaks in the insulation layer which would allow a significant amount of heat to ‘leak’ through to the living space 16. For example, a 2-ft by 4-ft non-insulated attic access door would allow 472 W (1613 BTU/hr) of heating to transfer to the living space 16 from a 140° F. attic 18. If a heat recovery device was used to bring the attic temperature down to 100° F., then this would reduce the heat transfer rate to 173 W (589 BTU/hr.). Another benefit of attic temperature reduction is increased service life for asphalt shingles due to the decrease in temperature to which they are exposed. In addition, the discomfort and hazard of working in an attic 18 during the warm part of the year is considerably reduced. Keeping the attic space 18 from reaching high temperatures also allows items to be stored in the attic 18 that would otherwise be damaged by high temperatures.

An example of early effort to exploit attic heat accumulation for warming a swimming pool 118 is R. David Burns' installation of finned copper tubes inside his attic 18 for the purpose of circulating pool water through them.

However, the success of this approach was severely limited due to an insufficient rate of convective heat transfer because no means of forced air movement was provided. The convective heat flux (heat transfer rate per unit area) is equal to the temperature difference between a surface and the fluid surrounding it times the convection coefficient. For air movement due only to thermally-induced buoyancy effects (free convection), the convection heat transfer coefficient (or film coefficient) falls in the range of 5 to 25 W/(m²·K). For forced convection where the air is propelled to move, the convection heat transfer coefficient will range between 25 and 250 W/(m²·K). Accordingly, the rate of convective heat transfer is strongly influenced by the velocity of the fluid (gas or liquid) wetting a heat exchanger surface. In addition to the influence of fluid velocity, effective heat transfer between a liquid and a gas requires much more surface area on the gas side of the heat exchanger 20. Edward G. Palmer introduced an attic-mounted compact air-to-water heat exchanger 20 with forced convection provided by a fan 22 placed at the face of the heat exchanger 20. The implementation of forced convection raised the heat transfer rate by a factor of 4 to 5 as compared to the Burns installation and also reduced the amount of heat exchanger material required. This development by Edward G. Palmer was patented and commercially developed as the SolarAttic brand model PCS1 and the later model PCS2.

The SolarAttic unit introduces water plumbing into the attic space 18 generally as a retrofit. Such an installation can be done successfully, but great care must be taken to prevent and contain water leaks that would cause substantial damage to the house below. The SolarAttic product does include features such as a water-tight pan and a float sensor to minimize the risk of a leak, but these features inevitably add to the cost of the product. Additionally, since the plumbing is routed through unconditioned space 18 in the attic, measures must be taken to prevent a freeze-induced rupture of the heat exchanger and its plumbing. The prospect of a damaging leak may be much more of a serious issue as a perceived risk by potential consumers rather than an actual risk. Potential customers may be particularly intimidated by SolarAttic's practice of sending the entire flow from a pool pump 34 (up to 3.47 liters per second or 55 gallons per minute) to an attic-mounted heat exchanger 20. Moreover, the necessary skill to achieve a leak-free installation on the first attempt would tend to exclude most consumers from being able to save money by installing the SolarAttic product themselves rather than hiring a professional. The chances of a damaging leak would depend more on the skill and care undertaken by the installer rather than the degree of manufacturing quality of the attic-mounted unit. Considering only the likelihood of a failure is a rather narrow evaluation of a risk or hazard. A more balanced assessment takes into account the severity of the damage resulting from a failure and if that damage can be afforded.

There is a need for improved systems for heating swimming pools and mitigating attic air temperatures to decrease the load on air conditioning.

SUMMARY

Embodiments of the present disclosure generally provide a pool heating system that extracts attic heat but does not allow any pool water into the living space of an associated house.

The pool heating system includes a pool containing a large volume of pool water and an associated building. The building having an attic space over a living space. A heat-exchanger is positioned outside of the living space of the building. The heat exchanger exchanges heat from the attic into pool water that is circulated through the heat exchanger. Any pool water that leaks from the heat-exchanger loop does not leak into or onto the living space.

Other technical features may be readily apparent to one skilled in the art from the following figures, descriptions and claims.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of this disclosure and its features, reference is now made to the following description, taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a schematic of an attic heat recovery device installed outside the footprint of the living space according to one embodiment of the present disclosure;

FIG. 2 is an exploded view of a fan-coil unit recessed into the overhang of a house according to one embodiment of the present disclosure;

FIG. 3 is an assembled view of a fan-coil unit recessed into the overhang of a house according to one embodiment of the present disclosure;

FIG. 4 shows how ducts may be installed on a soffit-mounted fan-coil unit to intake air from the hotter upper portion of an attic according to one embodiment of the present disclosure;

FIG. 5 shows how partitions may be assembled to form a duct from the upper part of an attic according to one embodiment of the present disclosure;

FIG. 6 shows an attic-mounted mixing fan along with a soffit-mounted fan-coil unit according to one embodiment of the present disclosure;

FIG. 7 is a liquid-to-liquid heat exchanger installed next to a house with the roof cut-away to reveal the attic-mounted fan-coil unit according to one embodiment of the present disclosure;

FIG. 8 is a detailed view of a liquid-to-liquid heat exchanger with pumps and plumbing according to one embodiment of the present disclosure;

FIG. 9 is a detailed view of a fan-coil mounted within an attic according to one embodiment of the present disclosure; and

FIG. 10 shows a heat pipe installed for attic heat recovery according to one embodiment of the present disclosure.

DETAILED DESCRIPTION

A practical design for preventing potential water damage is to install the air-to-water heat exchanger 20 in a location that is outside the footprint of the living space 16 of the house 56, but still exposed to hot attic air (FIG. 1). In other words, the heat exchanger 20 would be installed outside the periphery of the ceiling. For example, if a house 56 features overhangs, then a heat exchanger 20 and fan 22 could be installed within the space above the soffit panel 74 and below the roof 24 deck (FIGS. 2 and 3). Having the heat exchanger recessed into an overhang would provide easy access, and would tend to permit much shorter water lines 26 than an installation further into the interior of an attic 18. Moreover, the heat exchanger 20 and the fan 22 could be almost completely concealed behind a grill 36 or louvered panel. This configuration also allows the pool heating system to function as an attic ventilator when heating is not required. A factor potentially counteracting the effectiveness of this relatively low heat exchanger position is the natural tendency for temperature stratification within an attic 18. Due to buoyancy, less dense hot air tends to rise and collect in the high spaces in the attic 18. Accordingly, a fan-coil unit 88 mounted within the overhang may tend to draw-out the cooler air in the lower attic 18 and not significantly extract the hotter air at the top. However, there are ways to overcome this potential difficulty. First of all, a sufficiently powerful heat exchanger fan 22 (in terms of suction and volume) could overwhelm the effects of thermally induced buoyancy. A different approach is to install inlet ducts 138 in the attic to connect the fans 22 to the higher reaches of an attic 18 (FIG. 4). If the attic space 18 adjacent to the overhang is too tight to install a duct 138, then an alternative approach (FIG. 5) would be to install partitions 98 inside the attic 18 to effectively form an inlet duct to the fans 22 from a high point in the attic 18. A solution using a more active approach would be to install a secondary fan 102 within the attic space to stir the air and break-up the stratification (FIG. 6). Any attic inlets in close proximity to the heat recovery unit would need to be blocked to prevent a short air recirculation pattern from forming. Because water is a much more effective heat transfer medium than air it is not necessary to send the entire flow from the pool pump 34 to the air-to-water heat exchanger 20; a flow of 10 gallons per minute should be more than sufficient. A small auxiliary pump 62 is a practical means of conveying this rate of water flow to the heat exchanger.

If there is insufficient space for a heat exchanger outside the footprint of the living space 16, then an alternative approach to intrinsically reduce the risk of water damage is to substantially limit the liquid volume that may possibly leak into an attic 18. This may be accomplished by introducing a second heat exchanger 64 into the system (FIGS. 7, 8, and 9). The air-to-liquid heat exchanger 20 inside the attic 18 would be connected to a liquid-to-liquid heat exchanger 64 outside of the house 56. Swimming pool water would only flow through one side of the liquid-to-liquid heat exchanger 64. The liquid introduced into the attic space 18 would be an intermediate heat-transporting liquid. The closed loop between the air-to-liquid heat exchanger 20 and the liquid-to-liquid heat exchanger 64 would contain a minimal volume of fluid. At most, only several gallons could be leaked into the house because that is all the liquid in the loop. In contrast, several thousand gallons could, in theory, leak into a house 56 when the pool water is pumped directly to the heat exchanger 20 in the attic 18. Liquids are generally much more effective heat transfer mediums than gases such as air. Accordingly, only a small flow of liquid is needed to take-up the heat from a fairly large flow of air. For this reason, the attic flow loop would only require a relatively small pump 62 and small tubes 26 as compared to the pump 34 and tubes 58 serving the swimming pool 118 which are sized for the filtration requirements for the pool 118. The liquid-to-liquid heat exchanger 64 in FIGS. 7 and 8 is depicted as a shell and tube heat exchanger. The intermediary liquid flows through the tubes and the pool water flows within the shell surrounding the tubes. There are other types of liquid-to-liquid heat exchangers 64 that could fulfill the same function, such as a plate-to-plate heat exchanger. Another advantage to this approach is that the heat exchanger 20 in the attic 18 is no longer exposed to harsh swimming pool chemicals. The intermediary liquid could be chosen to allow the attic heat exchanger 20 to be made from inexpensive materials rather than copper, stainless steel or cupro-nickel alloys. Furthermore, the intermediary liquid could be selected based on its resistance to freezing and/or a tendency to quickly turn into vapor at atmospheric pressure. A prudent additional layer of protection could be provided by a catch tray 66 installed beneath the attic coil 20 and the lines 26 leading to it (FIG. 9). Alternatively, the liquid lines 26 could be enclosed within a larger tube which drains outside.

A variation on the approach described above would be to use a heat-pipe 120 to extract heat from the attic air (FIG. 10). A heat pipe 120 is a sealed tube containing a fluid in both a liquid and vapor state which behaves as if it were a highly heat conductive rod. Evaporation takes place at the hot end of a heat pipe 120 and condensation occurs at the cooler end. Convection heat transfer with a phase change yields a very high convection heat transfer coefficient. For convection involving condensation or evaporation coefficients typically range from 2500 to 100,000 W/(m²·K) as opposed to 250 to 20,000 W/(m²·K) for convection of a liquid without a phase change. Convection heat transfer is directly proportional to the temperature difference and the convection heat transfer coefficient. Another advantage of a heat pipe 120 is that it does not require a pump 62 to circulate the heat transfer fluid. The liquid is transported by capillary effects from an internal wick or finely grooved surface. However, gravity is often utilized to assist the flow of liquid to the evaporating section 124 of a heat pipe 120. A related device that lacks any wicking means and thereby depends completely on gravity to move the liquid to the evaporating end is sometimes refer to as a thermosiphon or a Perkins tube. So for an attic heat recovery application, the evaporating section 124 would have a fan blowing attic air over it, and would have fins on the heat pipe tubes to provide a large surface area. The condensing section 122 of the heat pipe would have swimming pool water flowing around it. If gravity is to assist transporting liquid to the evaporating end 124, the condensing end 122 of the heat pipe that exchanges heat with the swimming pool water would have to be installed well above the level of the attic floor. Otherwise a heat pipe 120 would only use capillary effects to transport the liquid to the attic end 124 of the heat pipe 120. FIG. 10 depicts a heat pipe 120 in a configuration that would also allow a Perkins tube to function. Typically only a small fraction of the fluid in a heat pipe 120 is in a liquid state. Moreover, a heat transfer fluid could be chosen that would completely evaporate at atmospheric pressure should the heat pipe tube develop a leak. Refrigerants are often used as heat pipe fluids.

The main purpose of the disclosed attic heat recovery system is to substantially lengthen the swimming season for a pool 118 at a minimal operating cost. Ideally, one would like to make the swimming season coincide with the time span of comfortable ambient air temperatures (greater than 18.3° C. or 65° F.). Beyond this period, when the weather is cool enough to warrant wearing long sleeves and/or a jacket, people are unlikely to be interested in swimming in an outdoor pool 118. A reasonable range for defining a comfort zone for swimming pool water is about 25.6 to 32.2° C. (78 to 90° F.). The temperature range of 25.6 to 27.8° C. (78 to 82° F.) perhaps could be described more accurately as tolerable rather than comfortable because it usually entails an initial chill when entering pool 118 followed by a brief period of getting accustomed to the water temperature. Accordingly, higher temperatures of 28.3 to 32.2° C. (83 to 90° F.) are much more pleasing because little or no initial chill is experienced.

The disclosed system is inherently less costly than solar panels due to a more efficient use of material since the large insolation surface is provided by an existing roof surface 24. In contrast to a fixed solar collector, the roof 24 of a house 56 is likely to have some substantial portion of its surface in direct sunshine during almost the whole course of a day. In this manner, an attic heat recovery device can exploit morning sun as well as afternoon.

In contrast to prior art air-to-water heat exchangers 20 mounted within an attic 18, the heat recovery device disclosed is much more forgiving to install. If it were to leak, the liquid would tend to fall outside the house 56 rather than soaking through the ceiling and flooding the living space 16. Accordingly, the skill level required for installation is within capabilities of a substantial fraction of home owners.

It may be advantageous to set forth definitions of certain words and phrases used in this patent document. The term “couple” and its derivatives refer to any direct or indirect communication between two or more elements, whether or not those elements are in physical contact with one another. The terms “include” and “comprise,” as well as derivatives thereof, mean inclusion without limitation. The term “or” is inclusive, meaning and/or. The phrases “associated with” and “associated therewith,” as well as derivatives thereof, may mean to include, be included within, interconnect with, contain, be contained within, connect to or with, couple to or with, be communicable with, cooperate with, interleave, juxtapose, be proximate to, be bound to or with, have, have a property of, or the like.

While this disclosure has described certain embodiments and generally associated methods, alterations and permutations of these embodiments and methods will be apparent to those skilled in the art. Accordingly, the above description of example embodiments does not define or constrain this disclosure. Other changes, substitutions, and alterations are also possible without departing from the spirit and scope of this disclosure, as defined by the following claims. 

1. A pool heating system comprising: a pool containing a large volume of pool water; a building with an attic space over a living space; a heat exchanger outside of the living space of the building that exchanges heat from the attic into pool water that is circulated through the heat exchanger such that any pool water that leaks from the heat exchanger does not leak into or onto the living space.
 2. The system of claim 1 wherein: the heat exchanger is an air-to-liquid heat exchanger.
 3. The system of claim 2 further comprising: a fan to direct warm attic air over the heat exchanger.
 4. The system of claim 2 wherein: the heat exchanger is located in a soffit of the attic outside of the living space of the building.
 5. The system of claim 4 further comprising: a fan to direct attic air over the heat exchanger.
 6. The system of claim 5 wherein: the fan is closely coupled to the heat exchanger.
 7. The system of claim 1 wherein: the heat exchanger is a liquid-to-liquid heat exchanger.
 8. The system of claim 7 further comprising: an air-to-liquid heat exchanger located in the attic space; and a heat transfer liquid running between the air-to-liquid heat exchanger and the liquid-to-liquid heat exchanger such that only the heat transfer liquid would enter the attic space.
 9. A pool heating system comprising: a pool containing a large volume of water; a building having a living space and an attic space, the attic space including soffit space that extends beyond the living space; an air-to-liquid heat exchanger located in the soffit space of the attic; a pump between the pool and the heat exchanger for pumping pool water through the heat exchanger.
 10. The system of claim 9 further comprising: a fan in the attic to direct air towards the heat exchanger.
 11. The system of claim 10 wherein: the fan is closely coupled to the heat exchanger.
 12. The system of claim 10 wherein: the fan is spaced from the heat exchanger.
 13. The system of claim 10 further comprising: duct work between the fan and the heat exchanger to direct attic air from the fan to the heat exchanger.
 14. A pool heating system comprising: a pool containing a large volume of water; a building with a living space and an attic space over the living space; heat pipes having a condensing section outside of the attic space and an evaporating section inside the attic space; and a pump between the pool and the condensing section of the heat pipe for pumping water from the pool to the condensing section of the heat pipe.
 15. The system of claim 14 further comprising: a fan in the attic space directing attic air over the evaporating section of the heat pipes.
 16. The system of claim 15 wherein: the fan is closely coupled to the evaporating section of the heat pipe. 